1. Field of the Invention
The present invention relates to a projection lens for a video projector which is used for projecting onto a large-size screen an image displayed on a projection tube (CRT) or the like; and, in particular, to a so-called optical coupling type (OC type) projection lens in which the space between the lens and CRT is filled with a liquid in order to increase contrast ratio or enhance cooling efficiency of the CRT.
2. Description of the Prior Art
Video projectors, by which an image displayed on a CRT surface or the like is projected onto a large-size screen by means of a projection lens, have often been used in theaters, exhibition halls, airplanes, and the like in recent years.
Preferably used as a projection lens for a video projector is that of OC type which is excellent in cooling efficiency and contrast ratio.
Also, the video projector is used as a terminal for displaying computer graphics images or the like as well as images from VTR, video cameras, and received TV images. It is required for the video projector to have a projection magnification of 10.times. to 60.times. and be inexpensive.
In an OC type projection lens, the liquid within its liquid optical coupler becomes quite hot due to the heat of the CRT surface, thereby causing the lens housing to expand. Consequently, the attaching position of the projection lens may vary, thus defocusing its projected image.
Therefore, a projection lens is proposed in commonly-assigned Japanese Unexamined Patent Publication No. 6-242372 as an optical coupling type projection lens which can correct the defocusing of projected image caused by increase in temperature and can secure a power-variable range of at least about 60.times. in the case where it is applied to a front-screen type projector.
The projection lens disclosed in Japanese Unexamined Patent Publication No. 6-242372 actively utilizes a plastic lens whose refractive index greatly varies upon increase in temperature, and adjusts the position and refractive index of the plastic lens so as to correct the defocusing of projected image caused by increase in temperature.
In this projection lens, however, since its temperature-correcting plastic lens has a power, in order to prevent lens performances from deteriorating due to changes in projection magnification, it is necessary to use other plastic lenses having a weaker power as well.
Accordingly, the projection lens as a whole comprises nine to ten lenses, thereby increasing its manufacturing cost.
Meanwhile, while the temperature of the liquid within the liquid optical coupler rises due to the heat of the CRT surface when the CRT is turned on as mentioned above, it has been known that the liquid temperature becomes substantially constant after it rises from room temperature by about 40.degree. C.
Accordingly, in the case where focusing is effected beforehand or again at the point of time when the temperature of the liquid within the liquid optical coupler becomes substantially constant, the temperature-correcting plastic lens can be omitted.